To make oneself appealing and successful with an audience, a potential employer, etc (1920s+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with sell oneself
Convince another of one's merits, present oneself in a favorable light, as in A job interview is an ideal opportunity to sell oneself to a prospective employer. Originally this idiom, dating from the second half of the 1700s, alluded to selling one's services for money, but it was being used more loosely by the mid-1800s.
Compromise one's principles for monetary gain. An early version was
sell oneself (or one's soul) to the devil, which alluded to enlisting the devil's help in exchange for one's soul after death. It is embodied in the legend of Faust, first recorded in the late 1500s.